Localised cerebral phosphorus-31 MR spectroscopy in man before and immediately after coronary bypass surgery with hypothermic cardiopulmonary bypass.

Coronary artery bypass surgery classically is undertaken with hypothermic cardiopulmonary bypass (CPB). There is a high incidence of neuropsychological defects after cardiac surgery, which may be related to cerebral ischaemia during the rewarming period. In this study, phosphorus-31 magnetic resonance spectroscopy (31P MRS) was used to identify changes in cerebral 31P MR spectra in patients before and immediately after hypothermic CPB. Four neurologically normal patients undergoing coronary artery bypass surgery were studied. Localised cerebral 31P MRS (TR 5000 ms) was performed at 1.5 Tesla on each patient the day before and within an hour of completion of surgery. Peak areas for phosphomonoesters (PME), inorganic phosphate (Pi), phosphodiesters (PDE), phosphocreatine (PCr) and beta ATP (betaATP) were measured. Metabolite peak area ratios and relative percentages of each 31P MR resonance with respect to the total 31P MR signal were calculated. In the post-operative MR spectra, each patient displayed a marked reduction in Pi/betaATP and increase in PCr/Pi ratios. Spectral changes in percentage metabolite signals following surgery varied both in magnitude and pattern between patients. In two patients there was an increased postoperative percentage PME and percentage PCr with a decrease in percentage betaATP. The converse was found in the other two patients, but all four subjects displayed a markedly decreased percentage Pi after CPB. These metabolite changes probably reflect rebound phosphorylation in the immediate postoperative period and suggest increased metabolic activity in the hyperaemic brain on rewarming from hypothermic CPB.